Voltage regulator with adaptation control

ABSTRACT

A voltage regulator coupled to an unregulated DC input voltage source by an input terminal, and to a load by an output terminal is disclosed. The voltage regulator converts an input voltage at the input terminal to an output voltage at the output terminal. The voltage regulator includes one or more slaves, and each slave includes a switching circuit. During each switching period of the switching circuit, the current for the slave is checked; namely, after the beginning of a low-side conduction period, and before the beginning of a high-side conduction period.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/540,493, filed Sep. 28, 2006, now issued as U.S. Pat. No. 7,522,436,which claims the benefit of priority from U.S. Provisional PatentApplication No. 60/722,249, filed Sep. 30, 2005, and from U.S.Provisional Patent Application No. 60/723,562, filed Oct. 3, 2005, thedisclosure of each of which is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

The present invention relates generally to voltage regulators, and moreparticularly to control systems for switching voltage regulators.

Voltage regulators, such as DC to DC converters, are used to providestable voltage sources for electronic systems, particularly electronicsystems that include integrated circuits. Efficient DC to DC convertersare particularly needed for battery management in low power devices,such as laptop notebooks and cellular phones, but are also needed forhigher power demand products, e.g., desktop computers or servers.Switching voltage regulators (or more simply “switching regulators”) areknown to be an efficient type of DC to DC converter. A switchingregulator generates an output voltage by converting an input DC voltageinto a high frequency voltage, and filtering the high frequency voltageto generate the output DC voltage. Typically, the switching regulatorincludes a switch for alternately coupling and de-coupling anunregulated input DC voltage source, such as a battery, to a load, suchas an integrated circuit. An output filter, typically including aninductor and a capacitor, is coupled between the input voltage sourceand the load to filter the output of the switch and thus provide theoutput DC voltage. A controller measures an electrical characteristic ofthe circuit, e.g., the voltage or current passing through the load, andsets the duty cycle of the switch in order to maintain the output DCvoltage at a substantially uniform level.

Voltage regulators for microprocessors are subject to ever morestringent performance requirements. One trend is to operate at everlower voltages, e.g., less than 1 volt, and at higher currents, e.g.,50-150 amps. Another trend is to turn on or off different parts of themicroprocessor in each cycle in order to conserve power. This requiresthat the voltage regulator react very quickly to changes in the load,e.g., several nanoseconds to shift from the minimum to the maximum load,and to have a fast transient response, e.g., to quickly stabilizewithout significant voltage or current ripple.

Still another trend is to place the voltage regulator close to themicroprocessor in order to reduce parasitic capacitance, resistanceand/or inductance in the connecting lines and thereby avoid currentlosses. However, in order to place the voltage regulator close to themicroprocessor, the voltage regulator needs to be small and have aconvenient form factor.

In addition to these specific trends, high efficiency is generallydesirable in order to avoid thermal overload at high loads and toincrease battery life in portable systems. Another desirable feature isfor the voltage regulator to have a “standby mode” which consumes littlepower at low loads.

SUMMARY OF THE INVENTION

A voltage regulator coupled to an unregulated DC input voltage source byan input terminal, and to a load by an output terminal is descried. Thevoltage regulator converts an input voltage at the input terminal to anoutput voltage at the output terminal. The voltage regulator includesone or more slaves, and each slave includes a switching circuit. Duringeach switching period of the switching circuit, the current for theslave is checked; namely, after the beginning of a low-side conductionperiod, and before the beginning of a high-side conduction period.

In some implementations, the voltage regulator includes an inputterminal coupled to an input voltage source and an output terminalcoupled to a load. The voltage regulator also can include a mastercontroller which detects an output voltage at the output terminal andgenerates a current control signal, a plurality of slaves, each slavehaving an intermediate terminal and receiving the current control signalfrom the master controller, each slave including a switching circuit toalternate between a first state in which the intermediate terminal isconnected to an input terminal and a second state in which theintermediate terminal is connected to a ground, and an internalcontroller configured to determine a first threshold based on thecurrent control signal and a first offset, receive a first measurementof an output current at the intermediate terminal, control the switchingcircuit to switch from one of the first state and the second state toanother of the first state and the second state when the output currentequals or exceeds the first threshold, determine a second thresholdbased on the current control signal and a second offset, adjust thefirst offset or the second offset based on a comparison of the firstmeasurement of the output current to the second threshold, and a filterbetween the intermediate terminals and the output terminal to provide agenerally DC output voltage at the output terminal.

In another implementations, a slave for the voltage regulator caninclude an intermediate terminal, a switching circuit to alternatebetween a first state in which the intermediate terminal is connected toan input terminal of the voltage regulator and a second state in whichthe intermediate terminal is connected to a ground, and an internalcontroller configured to receive a current control signal, determine afirst threshold based on the current control signal and a first offset,receive a first measurement of an output current at the intermediateterminal, control the switching circuit to switch from one of the firststate and the second state to another of the first state and the secondstate when the output current equals or exceeds the first threshold,determine a second threshold based on the current control signal and asecond offset, and periodically adjust the first offset based on acomparison of the first measurement of the output current to the secondthreshold.

Advantages of the invention may include the following.

The voltage regulator handles relatively large currents and reactsquickly to changes in the load. The voltage regulator can includemultiple slaves which are operated out of phase in order to reducecurrent ripple.

These and other features and advantages are realized by a method ofoperating a voltage regulator having an input terminal coupled to aninput voltage source, an output terminal coupled to a load and anintermediate terminal coupled to the input terminal and the outputterminal, the method including detecting an output voltage at the outputterminal, generating a current control signal based on the detectedoutput voltage, determining a first threshold based on the currentcontrol signal and a first offset, receiving a first measurement of anoutput current at the intermediate terminal, determining a secondthreshold based on the control signal and a second offset, comparing thefirst measurement of the output current to the second threshold, andselecting one of a first state and a second state to produce a desiredoutput current at the intermediate terminal.

Other objects, features, and advantages of the present invention willbecome apparent to one skilled in the art from the following detaileddescription and accompanying drawings illustrating features of thisinvention by way of example, but not by way of limitation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary switching regulator.

FIG. 2 is a block diagram of an exemplary switching regulator thatincludes a token ring communication system and a phase locked loop.

FIG. 3 is a timing diagram showing exemplary phase shifted switchingclock signals.

FIG. 4 is a block diagram of an exemplary slave that receives a desiredtotal current output control signal that is differential analog signal.

FIG. 5 is a graph illustrating determination of an exemplary desiredcurrent for a slave from a desired total current output control signal.

FIG. 6 is a timing diagram illustrating an exemplary switching clocksignal, a ramping function, a current passing through a slave, andconduction periods of the high-side and low-side power transistors of aslave.

FIG. 7 is a flow chart showing an exemplary method performed by slavesof an exemplary switching regulator.

FIG. 8 is a timing diagram illustrating an exemplary ramping function,current passing through a slave, and timing of measurements to determinean offset of a ramping function from a desired current.

FIG. 9 illustrates a conventional voltage regulator.

DETAILED DESCRIPTION

FIG. 9 illustrates a prior art voltage regulator 910. Referring to FIG.9, the switching regulator 910 includes one or more slaves 916 forconverting an input voltage V_(in) at the input terminal 920 to anoutput voltage V_(out) at the output terminal 922, and a mastercontroller 918 for controlling the operation of the slaves 916. Eachslave 916 includes current sensors 940 and 942 to measure the currentflowing through the transistors 930 and 932, respectively. Each currentsensor generates a digital output signal on one or more output lines.The digital output signal on the output line then switches from high tolow or from low to high depending on whether the current passing throughthe slave exceeds or falls below a trigger current. The mastercontroller 918 then incorporates such switching information from thecurrent sensors 940 and 942 in a digital current-based controlalgorithm. Specifically, based on the output voltage V_(out) and signalfeedback received from the slaves and the current sensors, the digitalcurrent-based control algorithm controls and sets each slave to anappropriate open/close state so that the output voltage V_(out) ismaintained at a substantially constant level. For example, a set ofcontrol signals a₁, a₂, . . . , a_(n), and b ₁, b₂, . . . , b_(i), onthe timing lines 944 a and 944 b generated by the master controller 918controls the open/close state of the transistors 930 and 932 in eachslave 916.

As noted above, each slave is individually controlled by itscorresponding control signals (e.g., switching signals) provided by themaster controller 918. Thus, one drawback associated with the prior artvoltage regulator 910 shown in FIG. 9 is that as the number of slavesincreases, the number of state control signals and state control linesfor driving each slave also increases proportionally. This translatesinto higher cost and larger size for the master controller 918.

Furthermore, the switching scheme of each slave is governed by themaster controller 918, and the received switching signals cannot belocally controlled or adjusted by the slaves. Thus, if the outputvoltage V_(out) at the output terminal 922 is modified, the mastercontroller 918 needs to alter the entire transistor switching schemepreviously assigned to the slaves so as to remain consistent with themodified output voltage. Such task lowers the overall efficiency of themaster controller 918, and possibly degrades the performance of thevoltage regulator 910.

Referring to FIG. 1, a switching regulator 10 is illustrated. Theswitching regulator 10 is coupled to an unregulated DC input voltagesource 6, such as a battery, by an input terminal 12. The switchingregulator 10 is also coupled to a load 8, such as an integrated circuitor video graphics board, by an output terminal 14. The load 8 typicallyhas an expected target load line voltage V_(target) and a voltagetolerance ΔV_(target). A typical target voltage V_(target) for amicroprocessor chip is in the range of about 0.5 to 1.6 volts, e.g.,about 1.0 volts, and a typical voltage tolerance ΔV_(target) is about+/−1-2% of the target voltage V_(target) e.g., about 15 mV over a loadline range of 0.8 to 1.2 volts. The switching regulator 10 serves as aDC-to-DC converter between the input terminal 12 and the output terminal14. The switching regulator converts an input voltage V_(in) at theinput terminal 12 to an output voltage V_(out) at the output terminal 14which is within the tolerance ΔV_(target) of the target voltageV_(target). Although the switching regulator will be illustrated anddescribed below for a buck converter topology, the invention is alsoapplicable to other voltage regulator topologies, such as boostconverter or buck-boost converter topologies.

The switching regulator 10 includes two or more (N) “slaves” 16 _(a), 16_(b), 16 _(c) and 16 _(d) and an equal number (N) of inductors 18 _(a),18 _(b), 18 _(c) and 18 _(d). Each inductor is coupled to the currentoutput of an associated slave 16 _(a), 16 _(b), 16 _(c) and 16 _(d).Although illustrated with four slaves, the switching regulator couldhave two, six, or another number of slaves. The switching regulator 10also includes a master controller 30 for controlling the operation ofthe slaves 16. The master controller 30 may be powered by the voltagesource 6 (as illustrated) or by another voltage source.

If the current drawn from the load 8 (or simply “load”) increases, thenthe amount of current passing through the slaves is increased. Thispermits the current to “ramp up” until the desired load is reached. Onthe other hand, if the current drawn from the load 8 decreases, theamount of current passing through the active slaves is also decreased.This permits the current to “ramp down” until the desired load isachieved. In general, the master controller 30 can provide command toincrease or decrease the current passing through the slaves 16 inresponse to a change in the output voltage after the load 8 changes. Insome implementations, each slave can internally “clamp” or make thenecessary adjustment to the current passing therethrough. That is, eachslave can be configured to adjust a variable ramp rate if it detectsthat its output current is too large.

Each slave 16 includes a switching circuit which serves as a powerswitch for alternately coupling and decoupling the input terminal 12 toan intermediate node 24 _(a), 24 _(b), 24 _(c) and 24 _(d). Theswitching circuit also includes a rectifier, such as a switch or diode,coupling the intermediate node 24 to a low voltage line, e.g., ground.The opening and closing of the switching circuit generates anintermediate voltage V_(int) having a rectangular waveform at theintermediate node 24. In particular, each slave can include a high-sidepower transistor 20 _(a), 20 _(b), 20 _(c) and 20 _(d) having a drainconnected to the input terminal 12 and a source connected to anintermediate node 24, and a low-side power transistor 22 _(a), 22 _(b),22 _(c) and 22 _(d) having a source connected to ground and a drainconnected to the intermediate node 24. Thus, each slave acts as aswitching circuit to alternate between coupling the intermediate node 24to the input terminal 12 and coupling the intermediate node 24 toground. In one implementation, both the high-side transistor 20 and thelow-side transistor 22 are N-type MOS (NMOS) devices (and the high-sidetransistor 20 can have a separate gate voltage supply with a groundreferenced to the source of the high-side transistor 20). In anotherimplementation, the high-side transistor 20 may be a P-type MOS (PMOS)device whereas the low-side transistor 22 may be an N-type MOS (NMOS)device. In still another implementation, the high-side transistor 20 maybe a P-type MOS (PMOS) device whereas the low-side transistor 22 may bean LD-type MOS (LDMOS) device. Moreover, in some implementations, thelow-side transistor 22 may be replaced or supplemented by a diode toprovide rectification. The two transistors 20 and 22 may be driven byswitching signals on control lines 40 and 42, respectively, to create arectangular voltage waveform at the intermediate node 24.

The intermediate nodes 24 of the slaves are coupled to the outputterminal 14 by an output filter. The output filter converts therectangular waveforms at the intermediate nodes 24 into a substantiallyDC output voltage at the output terminal 14. The output filter can beconsidered to include both the inductors 18, and a capacitor 38 that isconnected to ground and to the output terminal 14 in parallel. Theinductors 18 and capacitor 38 are discrete elements that can be securedto the same board as the chip with the master controller 30 and thechips with the slaves 16.

In any particular slave, when the first transistor 20 is closed and thesecond transistor 22 is open (the high-side conduction state), theintermediate node 24 is connected to the voltage source 6 and thevoltage source 6 supplies energy to the load 8 and the associatedinductor 18 through the first transistor 20. On the other hand, if thefirst transistor 20 is open and the second transistor 22 is closed (thelow-side conduction state), the intermediate node 24 is connected to thelow voltage line, e.g., ground, and the energy is supplied to the load 8by the inductor 18.

Each slave 16 can be fabricated on a separate chip. The mastercontroller 30 can be fabricated on a separate chip from the slaves 16,or be located on the same chip as one of the slaves. The mastercontroller 30 and the slaves 16 can be constructed with a combination ofanalog and digital components.

The master controller 30 sets a switching frequency (S-phase) and adesired total output current (I-desired) on control lines 32 and 34,respectively. The master controller 30 can be connected to voltagesampling circuitry that measures the output voltage V_(out) at theoutput terminal 14 at one or more discrete times during each cycle ofthe switching circuit. The sampling circuit may be constructedsubstantially as described in U.S. Pat. No. 6,020,729, the entiredisclosure of which is incorporated herein by reference.

The master controller 30 includes control circuitry, e.g., analogcircuitry, which can use the measured output voltage V_(out) todetermine a desired total current which will maintain the output voltageV_(out) substantially at the desired target voltage V_(target), e.g.,within the voltage tolerance. The desired total current is output as theI-desired signal on a control line 34, e.g., as an analog signal.Optionally, the master controller 30 can also use information from theslaves, e.g., sent over a communication ring 50 as will be describedbelow with respect to FIG. 2, to determine the desired total current.

In some implementations, the target voltage V_(target) can be a functionof the load 8, and can include a droop voltage. The target voltageV_(target) can be a linear function of the load 8, e.g. a load linevoltage described as V_(target)=V_(nom)−I_(load)×R_(droop), whereV_(nom) is the nominal voltage at zero current draw, I_(load) is thecurrent flowing into the load, and R_(droop) is a constant representinga droop resistance. In these implementations, to determine the desiredtotal current, the master controller 30 can make a simple differentialcomparison between V_(nom) and V_(out), and then multiply the differenceby a gain factor G1, i.e., I_(desired)=G1×(V_(nom)−V_(out)). Based onthe target voltage V_(target), the switching regulator 10 can adjusteach slave to output a desired current that maintains the output voltageV_(out) at substantially the target voltage V_(target). This techniqueensures that the slaves adapt to changes with respect to the outputvoltage V_(out) that may deviate from the target voltage V_(target). Inother implementations, the target voltage V_(target) can be a constantvalue, independent of the load 8. In this later case, the mastercontroller 30 would use a different technique to determine the desiredtotal current.

In some implementations, the slaves receive a differential analog signal(e.g., differential voltage signal) from the master controller 30, andre-interpret the differential analog signal as an average currentcommand. The slaves can be configured to output an average currentproportional to the differential analog signal. The slaves canimmediately detect if the average current command is high enough thatcould potentially damage the switching regulator 10, and effectivelyclamp the average current output so that a constant average current(e.g., an amount of current lower than the average current command) isoutput.

To determine and output an average current, the slaves can includeinternal circuitry that receives the differential analog signal, andconverts the differential analog signal to a current command signal. Forexample, the internal circuitry can generate a high current commandsignal in response to a high differential voltage signal. The incomingdifferential voltage signal is then monitored on a continuous basis, andif the incoming differential voltage signal is deemed too high, theslaves can clamp the current command signal so as to keep the outputcurrent constant. The level to which the current command signal isclamped can be pre-configured as a default value in the slave, or can becommunicated to each slave through a communication ring as will bedescribed below with respect to FIG. 2.

In addition, the master controller 30 can include a clock that generatesthe S-phase signal, which is output on a control line 32. The S-phasesignal is typically a square wave, and can have a frequency in the rangeof 500 kHz to 1500 kHz, e.g., 800 kHz. The S-phase and I-desired signalswill be discussed in greater detail below.

Each slave 16 also includes an internal controller 26 which sendscontrol signals on the control lines 40 and 42 as shown in FIG. 2 to thegates of the high-side transistor 20 and low-side transistor 22. Eachinternal controller 26 receives the same signals from the common controllines 32 and 34 (see FIG. 1) from the master controller 30. In addition,although not illustrated, each slave includes circuitry to measure theoutput current from the slave that flows into the associated inductor18, e.g., the current output at intermediate node 24, and to providethis information to the internal controller 26. In some implementations,the internal controller 26 can include an internal circuitry similar tothat discussed above for determining and outputting an average current.

Based on the control signals 32, 34 and the measured current, eachinternal controller 26 determines when to switch between the high-sideand low-side transistors in order to achieve, on average, the desiredcurrent for the particular slave, as will be discussed in greater detailbelow. In addition, the internal controller 26 ensures that thehigh-side transistor 20 and low-side transistor are not conductingsimultaneously.

The intermediate node 24 of each slave 16 is electrically connected toone end of an associated inductor 18, whereas the other end of eachinductor is electrically coupled and connected to the output terminal14. In particular, the inductors 18 can be coupled (e.g., wound around acommon core), and each winding can be made in the same orientation. Thecoupled inductors can be designed to have a ratio of magnetizinginductance to leakage inductance (LM/LL) of 7 to 10. In addition, theinductors 18 can have a magnetizing inductance sufficiently low toaccommodate a DC current difference of about five amperes between theinductors without saturation (and instantaneous differences can belarger without saturation). Alternatively, the inductors 18 could beuncoupled.

One end of the capacitor 38 is connected to a ground and the end otherend of the capacitor 38 is connected to one end of the output terminal14. The same end of the output terminal 14 is in parallel with theinductors 18. Thus, the combination of the inductors 18 and thecapacitor 38 converts the rectangular waveforms of the voltage at theintermediate nodes 24 into a substantially DC output voltage at theoutput terminal 14.

Referring to FIG. 2, in addition to receiving the control signals oncontrol lines 32, 34, the internal controllers 26 and the mastercontroller 30 can communicate using a ring communication scheme. Thatis, each controller (including each internal controller 26 and themaster controller 30) is connected sequentially in a communication ring50, and can be configured with a ring communication protocol to passdata bits sequentially from controller to controller (the communicationring could include a single line, such that the ring can only operateserially). The connections between the input and output ports on theslave chips can be hardwired electrical leads on the board.

In brief, the communication ring 50 can operate by passing messageframes, with data or commands, from one controller to another. Themessage frame can include a header identifying the intended recipientcontroller of the message. For example, a command generated by themaster controller 30 can be passed from the master controller 30 to theimmediately adjacent internal controller. That internal controller 26determines whether the message is addressed to it. If it is not theintended recipient, the internal controller 26 passes the message to thenext controller. Thus, the message is passed from controller tocontroller until it reaches the addressed controller. If the internalcontroller 26 is the intended recipient, the internal controller 26 canact on the message, e.g., by sending a message out from its output port,for example, along the sequence of remaining controllers back to themaster controller. In addition, each message is essentially a token thatrepresents which controller is permitted to transmit messages, thusavoiding fighting over control of the communication ring 50. In otherimplementations, the communication ring 50 could be replaced by a tokenring, or by an Ethernet, to permit communication between thecontrollers.

This ring-communication scheme can be used to pass commands from themaster controller 30 to the internal controllers 26 and to passinformation from the internal controllers 26 back to the mastercontroller 30. Unlike the S-phase and I-desired signal, which are usedfor immediate control of the response of the power transistors, thesecommands and information are of lower priority tasks, e.g., configuringthe slaves, that do not require action at the S-phase frequency. Forexample, switching events triggered by the S-phase signal can beseparated by about 1.25 microseconds (assuming that the S-phase signalhas a frequency of 800 kHz). In contrast, the messages passed throughthe communication ring may have an expected response time severalhundred to thousand times slower, e.g., on the order of 300 microsecondsto 1 millisecond.

Each internal controller 26 includes a phase-locked loop 60 whichoffsets the S-phase signal on line 32 so that each slave is phase-offsetrelative to the other slaves. The phase-offset S-phase signal is inputto the control circuitry 62 in the internal controller 26 so that thephase onset of the high-side conduction phase of each slave is phaseoffset from the other slaves. For example, if two slaves are active,then they should be 180° out of phase, and the time delay should beequal to one-half of the switching period T, i.e., Φ(1)=½T, of theS-phase signal. As shown in FIG. 3, if four slaves are active, then theyshould be 90° out of phase, and the time delays Φ(1), Φ(2), Φ(3) shouldbe equal to one-third and two-thirds of the switching period,respectively. More generally, for N slaves, the time delay should be 0,1/N . . . , (N−1)/N of the switching period T. However, under somecoupled inductor configurations, the phase shift may not be (N−1)/N ofthe switching period. In general, by operating the slaves out of phase,the current ripples from each slave will at least partially cancel,thereby providing a more constant output current from the switchingregulator.

The phase-offset for the phase-locked loop 60 can be set during astart-up configuration. In particular, during start-up configuration forthe voltage regulator 10, the master controller 30 sends a command onthe communication ring 50 that counts the number of slaves and causeseach slave to determine what particular phase offset should be used forthat slave. For example, the master controller can generate a start-upconfiguration message that includes a counter. After receiving themessage, the slave 16 increments the counter, records the number of thecounter in an internal register, and passes the message to a next slave16. When the last slave passes the message back to the master controller30, the counter is equal to the number of slaves, and each slave has aunique identification in its register. Then the master controllergenerates a second message on the communication ring 50 that sends thevalue of the total number of slaves to each internal controller 26.

Since each internal controller then has a unique identifier and knowsthe total number of slaves, it can determine the appropriate phaseoffset for itself, e.g., by accessing a look-up table, e.g., as storedin a custom digital circuit or in the memory of a controller. Forexample, the internal controller can simply calculate a phase offsetΦ(i) representing the time delay in phase lock loop 60 using thefollowing table:

TABLE 1 Desired phase Total number of active slaves offset 1 2 3 4 5φ(0) [reference] 0 0 0 0 0 φ(1) ½ T ⅔ T ½ T ⅗ T φ(2) ⅓ T ¼ T ⅕ T φ(3)¾ T ⅘ T φ(4) ⅖ T

In general, as shown in the Table, the system can allocate switchingslots so that as few spatially adjacent slaves (e.g., based on positionsof the inductors on the core) as possible have adjacent switchingevents. For example, the sum of phase differences between spatiallyadjacent slaves can be maximized.

It is also possible for there to be more complex arrangement of theinductors, which can result in a more complex phase offsetdetermination. For example, it is possible for the switching regulatorto include two or more inductor banks, with each bank of inductors woundaround its own core. For example, a switching regulator can include sixinductors, with a first bank of three inductors coupled to each other(e.g., wound around a first core), a second bank of three inductorscoupled to each other (e.g., wound around a separate second core), butthe first group not coupled to the second group. In general, if thebanks include an equal number of inductors, then the slaves can beoffset by 0, 1/N . . . , (N−1)/N of the switching period T, where N isthe total number of slaves. For example, in the example with two banksof three inductors, the first bank could have phase offsets of 0, 120°and 240° and the second bank could have phase offsets of 60°, 180° and300°.

It is also possible for the two banks to have different numbers ofinductors. In this case, the proper phase offset can depend on whetherthe goal is to minimize slave current ripple or to minimize aggregateoutput current ripple. In general, if the goal is to minimize aggregateoutput current ripple, the slaves are switched as if uncoupled, e.g.,the slaves are phase offset by 0, 1/N . . . , (N−1)/N of the switchingperiod T, where N is the total number of slaves.

On the other hand, if the goal is to minimize slave current ripple, theslaves within a given bank are phase offset by 0, 1/M . . . , (M−1)/M ofthe switching period T, where M is the number of slaves in the givenbank. In addition, the first slave in one bank can be phase offset fromthe first slave in the second bank. For example, for a switchingregulator that includes a first bank of five inductors and a second bankof four inductors, the inductors of the first bank can switch at 0, 72°,144°, 216° and 288°, and the second bank of inductors can switch at 45°,135°, 215° and 285°. The exact phase differences will depend on thegranularity of the phase locked loop 60. Alternatively, one or more ofthe particular slaves can be phase offset to avoid switching at the sametime as another slave.

Although illustrated as a single line 34 in FIG. 1, the I-desired signalcan be a differential analog signal as shown in FIG. 4. In particular,the master controller 30 can output a first analog voltage signalV_(I-desired-high) on line 34 a, and a second analog voltage signalV_(I-desired-low) on line 34 b.

The internal controller 26 can include a differential amplifier 52 togenerate a voltage differenceV _(I-desired) =V _(I-desired-high) −V _(I-desired-low)

and an averaging node 54 to calculate the common mode voltageV _(common mode)=(V _(I-desired-high) +V _(I-desired-low))/2

Referring to FIG. 5, the differential amplifier 52 (or other elements incircuitry 62) is configured to output a voltage that represents thedesired current I_(des)(i) for the particular slave. The desired slavecurrent I_(des)(i) represents a value proportional to the desired totalcurrent, e.g., 1/N (where N is the number of slaves) of I-desired, butis capped at a maximum current I_(max). (represented by a maximumvoltage). Capping the desired slave current provides over-currentprotection to prevent the voltage regulator from attempting to output anamount of current that could damage the circuit. Both the gain G2 andthe maximum current value I_(max) can be programmable. I_(max) and G2can be set by commands passed from the master controller 30 through thecommunication ring 50.

As noted above, the internal controllers 26 receive the S-phase signalon control line 32 and I-desired signal on control line 34, andindependently determine whether to switch between the high-side andlow-side conduction phases. However, the common mode voltage can also beused to override the internal controllers 26. If the common mode voltageV_(common mode) falls within a preset range (e.g., between a firstthreshold and a second higher threshold), the internal controller 26operates normally, and determines locally whether to switch between thehigh-side and low-side conduction phases. However, if the common modevoltage V_(common mode) is outside the preset range (e.g., either belowthe first threshold or above the second threshold), switching isdisabled and the slave is forced to the low-side conduction phase (oralternatively to a floating state or high impedance state in which bothtransistors are open). This is useful if a sudden shut-down of the slaveis needed.

Referring to FIG. 6, at a given slave, the internal controller 26triggers the high-side conduction period based on the calculated phaseshift from the leading edge of the S-phase signal 32. Thus, barringperiod-skipping events, the switching circuit has a frequency equal tothe S-phase signal, and a given slave switches to the high-sideconduction period at each leading edge of the S-phase signal (as phaseshifted for the particular slave).

The internal controller 26 triggers the low-side conduction period basedon a comparison of an output current from the slave I_(slave) to aramping function I_(ramp). Specifically, if the output current I_(slave)equals or exceeds the ramping function I_(ramp), the slave switches tothe low-side conduction phase. The measurement of the output current canbe provided by a current mirror that mirrors the current flowing fromthe intermediate node 24 to the inductor 18.

The slope of the ramping function I_(ramp) can affect the slave's outputresponse to changes in the average current commands. Those skilled inthe art will understand that the relationship between the value of theslope of the ramping function, the output voltage and the values of theinductors interact to provide different dynamics.

In another implementations, the internal controllers 26 can generate anestimated or ghost current that represents the actual slave current, andthe estimated current can be compared to the ramping function. Theestimated current can be calculated by increasing the estimated currentI_(estimate) by a ramp-up value ΔI_(up) at each clock cycle during thehigh-side conduction state, and decreasing the estimated currentI_(estimate) by a ramp-down value ΔI_(down) at each clock cycle duringthe low-side conduction state. Ramp-up and ramp-down values arediscussed in U.S. Pat. No. 6,268,716, the entire disclosure of which isincorporated by reference in its entirety. The estimated current can beperiodically compared to the actual slave current to adjust theestimate.

The ramp function I_(ramp) can be generated by analog components or bedigitally synthesized. Each slave can include an internal circuitry(e.g., internal controller 26) to provide the values of the rampfunction I_(ramp). In these implementations, the slave can providemultiple values for the slope of the ramp function I_(ramp) so as toaccommodate various switching regulators designed for differentapplications. Each slave can employ a different ramp function, and thecommunication ring 50 can be used to inform each slave a suitable slopefor a corresponding ramp function selected for that particular slave.

At the start of each high-side conduction period (e.g., as triggered bythe leading edge of the phase-shifted S-signal from the phase lock loop60), the ramp function I_(ramp) is set to a value equal to the desiredcurrent I_(des) plus an offset ΔI_(offset). The ramp function thendecreases with a slope G3. The value for G3 can be set digitally and canbe passed from the master controller 18 to the internal controllers 26using the communication ring 50.

If the desired total current I_(desired) increases, the desired slavecurrent I_(des) also increases, thus raising the ramp function I_(ramp).As a result, the high-phase conduction period will last longer (at leastduring that cycle), and the current will ramp up for a longer time (atleast during that cycle), and average slave current will increase.Conversely, if the desired total current I_(desired) decreases, thedesired slave current I_(des) also decreases, thus lowering the rampfunction I_(ramp). As a result, the high-phase conduction period will beshorter (at least during that cycle), the current will ramp down for alonger time (at least during that cycle), and the average slave currentwill decrease.

The internal controller 26 can include a combination of analog anddigital components. For example, the internal controller can includeanalog components that generate the ramp function and compare themeasured output current to the ramp function to determine whether toswitch the transistors to the low-side conduction state. However,various control voltages for the analog components can be set by digitalcomponents.

Referring to FIG. 7, the internal controller determines a desiredcurrent I_(des) for the particular slave, e.g., by applying a gain tothe differential analog signal I-desired 34 (step 102), generates a rampcurrent I_(ramp) for the slave based on I-desired (step 104), controlsthe high-side transition of the first and second transistors 20 and 22based on the S-phase signal (step 106) and controls the low-sidetransition of the first and second transistors 20 and 22 by a comparisonof the slave current to the ramp function (step 108), e.g., when theestimated output current I_(estimate) equals or exceeds the rampingfunction I_(ramp). It should be realized that, as these steps can beperformed by analog components, where appropriate they can be consideredto be performed on a continuous and simultaneous basis.

The internal controller 26 can also includes a microcontroller 66 (seeFIG. 4) that can perform a digital control algorithm at a clockfrequency f_(slave-clock). The clock frequency f_(slave-clock) can begenerated by multiplying the S-phase signal on line 34, e.g. by a factorof ten to fifty, e.g. by twenty-eight or thirty. The resulting clockfrequency f_(clock) can be between about 16 and 66 MHz, e.g., about 33MHz. Alternatively, the clock signal f_(slave-clock) may be generated bythe same clock that runs the microprocessor, by other IC devices in theload, or by a clock on the master controller chip.

The offset ΔI_(offset) for the ramp function is selected such theresulting switching performance causes the average current from theslave to be about equal to the desired slave current I_(des). However,the inductance L, the input voltage V_(in), are not known exactly (inaddition, the output voltage V_(out) is not necessarily known in theinternal controller 26) and can change over time or vary from circuit tocircuit, causing the slope of the slave current to depart from theexpected value. Thus, the offset ΔI_(offset) can depart from the valueneeded to cause the average current from the slave to be about equal tothe desired slave current. However, it is possible to use currentmeasurements in an algorithm to correct the offset ΔI_(offset).

Referring to FIG. 8, in one implementation, each switching period T, thecurrent I_(slave) for the slave is checked twice. First, at a time t1, afixed time Δt after the beginning of the low-side conduction period, theactual current I_(slave1) is compared to the desired current I_(des)plus a current offset DELTA. Second, at a time t2 a fixed time Δt beforethe beginning of the high-side conduction period, the actual currentI_(slave2) is compared to the desired current I_(des) minus the currentoffset DELTA. Thus, 2×DELTA is about equal to the peak to valleydifference of the estimated current.

In some implementations, the phase-locked loop 60 can generate an outputsignal having a higher frequency than the S-phase signal on line 32. Thesignal having a higher frequency is then used to configure the fixedtime Δt, current offset DELTA, time t1 and time t2 to improve theefficiency (e.g., the switching period and phase difference of theslaves) of the switching regulator 10.

If the first current I_(slave1) is greater than I_(des)+DELTA and thesecond current I_(slave2) is greater than I_(des)−DELTA, then it islikely that the overall current is too high, and therefore the offsetΔI_(offset) can be reduced. If the first current I_(slave1) is less thanI_(des)+DELTA and the second current I_(slave2) is less thanI_(des)−DELTA, then it is likely that the overall current is too low,and therefore the offset ΔI_(offset) can be increased. If the firstcurrent I_(slave1) is greater than I_(des)+DELTA but the second currentI_(slave2) is less than I_(des)−DELTA, then it is likely that the peakto peak current is larger than DELTA, and therefore DELTA can beincreased. If the first current I_(slave1) is less than I_(des)+DELTAbut the second current I_(slave2) is greater than I_(des)−DELTA, then itis likely that the peak to peak current is smaller than DELTA, andtherefore DELTA can be decreased. This is summarized in the followingtable:

TABLE 2 comparison at t1 comparison at t2 result I_(slave1) > I_(des) +DELTA I_(slave2) > I_(des) − DELTA decrease ΔI_(offset) I_(slave2) <I_(des) − DELTA increase DELTA I_(slavel) < I_(des) + DELTA I_(slave2) >I_(des) − DELTA decrease DELTA I_(slave2) < I_(des) − DELTA increaseΔI_(offset)

Once both DELTA and the ramp offset ΔI_(offset) have the proper values,the currents at t1 and t2 will be equidistant from the desired currentI_(des), and consequently average of the real current I_(slave) willmatch the desired slave current I_(des).

1. A voltage regulator, comprising: a switching circuit to alternatebetween a first state in which an intermediate terminal is connected toan input terminal and a second state in which the intermediate terminalis connected to a ground; an output terminal coupled to the intermediateterminal; control circuitry configured to measure an output voltage atthe output terminal, determine a control signal based on the outputvoltage, determine a first threshold based on the control signal and afirst offset, make a first measurement of an output current at theintermediate terminal, control the switching circuit to switch from oneof the first state and the second state to another of the first stateand the second state when the output current crosses the firstthreshold, determine a second threshold based on the control signal anda second offset, and adjust the first offset or the second offset basedon a comparison of the first measurement of the output current to thesecond threshold; and a filter coupled to the switching circuit toprovide a generally DC output voltage at the output terminal.
 2. Thevoltage regulator of claim 1, wherein the control circuitry includes amaster controller configured to measure the output voltage at the outputterminal and determine the control signal based on the output voltage.3. The voltage regulator of claim 2, further comprising a slave, whereinthe control circuitry includes an internal controller in the slave. 4.The voltage regulator of claim 1, wherein the control circuitrygenerates a clock signal and the control circuitry is configured tocontrol the switching circuit to switch from the second state to thefirst state at most once per clock cycle of the clock signal.
 5. Thevoltage regulator of claim 4, wherein the control circuitry isconfigured to control the switching circuit to switch from the secondstate to the first state at a rising edge of the clock signal.
 6. Thevoltage regulator of claim 1, wherein the control circuitry isconfigured to control the switching circuit to switch from the firststate to the second state when the output current crosses the firstthreshold.
 7. The voltage regulator of claim 1, wherein the firstthreshold is a ramp function.
 8. The voltage regulator of claim 1,wherein the control circuitry is further configured to: make a secondmeasurement of the output current at the intermediate terminal;determine a third threshold based on the control signal and the secondoffset; and periodically adjust the first offset or the second offsetbased on a comparison of the second measurement of the output current tothe third threshold.
 9. The voltage regulator of claim 8, wherein thecontrol circuitry adds the second offset to the desired slave current todetermine the second threshold, and subtracts the second offset from thedesired slave current to determine the third threshold.
 10. The voltageregulator of claim 9, wherein the first measurement is performed at apredetermined time after switching from the first state to the secondstate and the second measurement is performed at a predetermined timebefore switching from the second state to the first state.
 11. Thevoltage regulator of claim 10, wherein the first offset is adjusted ifthe first measurement and second measurement have a similar relationshipto the second threshold and third threshold, respectively.
 12. Thevoltage regulator of claim 10, wherein the second offset is adjusted ifthe first measurement and second measurement have a similar relationshipto the second threshold and third threshold, respectively.
 13. Thevoltage regulator of claim 10, wherein the first offset is increased ifthe first measurement is less than the second threshold and the secondmeasurement is less than the third threshold.
 14. The voltage regulatorof claim 10, wherein the first offset is decreased if the firstmeasurement is greater than the second threshold and the secondmeasurement is greater than the third threshold.
 15. The voltageregulator of claim 10, wherein the second offset is increased if thefirst measurement is greater than the second threshold and the secondmeasurement is less than the third threshold.
 16. The voltage regulatorof claim 10, wherein the second offset is decreased if the firstmeasurement is less than the second threshold and the second measurementis greater than the third threshold.
 17. The voltage regulator of claim10, wherein the first offset is increased if the first measurement isless than the second threshold and the second measurement is less thanthe third threshold, the first offset is decreased if the firstmeasurement is greater than the second threshold and the secondmeasurement is greater than the third threshold, the second offset isincreased if the first measurement is greater than the second thresholdand the second measurement is less than the third threshold, and thesecond offset is decreased if the first measurement is less than thesecond threshold and the second measurement is greater than the thirdthreshold.
 18. The voltage regulator of claim 1, wherein the controlsignal represents a desired current.
 19. A slave comprising: a switchingcircuit coupled to an output terminal through an intermediate terminal;and control circuitry configured to: determine a control signal based onan output voltage at the output terminal, determine a first thresholdbased on the control signal and a first offset, make a first measurementof an output current at the intermediate terminal, control the switchingcircuit based on when the output current crosses the first threshold,determine a second threshold based on the control signal and a secondoffset, and adjust the first offset or the second offset based on acomparison of the first measurement of the output current to the secondthreshold.
 20. A method comprising: determining a control signal basedon an output voltage at an output terminal of a circuit; determining afirst threshold based on the control signal and a first offset;measuring an output current at an intermediate terminal of the circuit;controlling a switching element associated with the circuit based onwhen the output current crosses the first threshold; determining asecond threshold based on the control signal and a second offset; andadjusting the first offset or the second offset based on a comparison ofthe measured output current to the second threshold.